In the melt extrusion of polymer resins there are often flow regimes, determined by the rheological properties of the particular resin, wherein anomalous flow behavior occurs, leading to imperfections on the extrudate surfaces. Such imperfections, commonly called melt fracture, appear in different forms. The so-called "sharkskin" fracture occurs at lower shear rates and appears as a general, finely-structured and uniform roughness. In a blown-film extrusion, sharkskin fracture may appear as an undesirable herringbone pattern, reducing clarity and giving a dull surface. In practice this may occur at uneconomically low extrusion rates. At higher shear rates flow often becomes unstable and a non-uniform stick-slip melt fracture results, wherein alternating bands of glossy surface and sharkskin fracture appear. This behavior is especially undesirable in wire coating and in tube and pipe extrusions as well as in blown-film applications.
In order to improve the extrusion behavior of hydrocarbon polymer resins through metal dies it is well known, for example, as disclosed in U.S. Pat. No. 3,125,547, to incorporate a small quantity of a fluoropolymer that is melted or above its glass transition temperature at the melt processing temperature, usually 120.degree.-300.degree. C. Such a process aid not only significantly reduces melt fracture but it also gives significant reduction in die pressure, thus providing cost-saving reductions in extruder power requirements. Widely used, commercially available process aids of this type are copolymers of vinylidene fluoride and hexafluoropropylene having Mooney Viscosities of 30-40 measured at 121.degree. C.
Japanese Kokai 1,074,247 discloses the use of combinations of fluoropolymers melting at temperatures below 100.degree. C together with fluoropolymers meltinq in the range 120.degree.-300.degree. C.
U.S. Pat. No. 4,904,735 discloses a process aid for a difficultly melt-processible (DMP) polymer, the process aid comprising, in combination, a fluorocarbon copolymer that, at the process temperature, is above its melting point or glass transition temperature together with a tetrafluoroethylene homopolymer or copolymer that, at the process temperature, is below its melting point and is solid.
Although not wishing to be legally bound by this explanation, it can be postulated that the fluorocarbon copolymer forms a thin lubricating layer at the die surface, over which the DMP polymer melt can flow without sticking, thus eliminating the undesirable melt fracture. However, in commercial extrusions it is often a practice to add to the DMP polymer an abrasive inorganic adjuvant (filler), for example, up to 1 wt. % of one or more pigments, or a finely divided talc or silica as an antiblock agent. This practice can greatly diminish, or completely negate, some or all of the beneficial effects achieved by means of the aforesaid combination process aid. It can be further postulated that the abrasive agent physically scrubs away the lubricating layer of fluorocarbon copolymer that is relatively weakly adhered to the die surface.
It is an object of this invention to provide a process aid that is resistant to the negative effects of abrasive inorganic fillers.